The need to measure of a length of conductive materials such as wires, cords, and ribbons is commonplace. It may be useful to know the length of a material that has been extended across a span, or to know the change in length of the material due to straining or stretching of the material itself. Many sensing techniques to determine length of such materials are demonstrated in the prior art which require secondary devices which sense the length or change in length. While many of these provide accurate measurements and various useful features, there are drawbacks to such devices. Typically, such devices increase the size, mass, and cost of the entire system and can be intrusive to the functionality of the material. It is desirable to determine the length of materials without such external sensors. Devices in the prior art have sought overcome these drawbacks by measuring properties of the material itself which might vary with changes in length or strain. More specifically, it is inexpensive and unobtrusive to measure the electrical properties of the material to sense such a change. Examples of this concept are demonstrated in prior art. For instance, U.S. Pat. No. 3,922,789 measures the electrical resistance of a wire payed-out from a reel and uses this measured electrical resistance to determine the wire length. Nevertheless, there continue to be drawbacks in determining length of conductive materials, as these methods cannot compensate for the elastic strain of the wire due to stress or changes in diameter due to wear.
Shape Memory Alloys (SMA) are special alloyed metals that undergo a large change in crystal geometry when heated and cooled that causes the alloy to expand and contract with large force. Shape memory alloys are utilized as solid-state actuators in many applications. These actuators have benefits including silent operation, high strength-to-weight ratio, and direct linear motion. Because the material models are very complex, the feedback control of SMA actuators is difficult and SMA actuators have not become prevalent in robot or machine design except in places where binary (two-position) motion is all that is needed. This binary control allows the model to be as simple as two known positions; the state when hot and when cold. However, a more useful actuator is one that is easily commanded to obtain a desired position anywhere within its range of capability, not just at set limits. To achieve this, generally a position sensor is used in a feedback loop with the actuator. While sensor feedback is feasible in many cases, the great benefits of SMA actuators (high payload-to-weight ratios, simplicity, and small size) cannot be realized in this configuration because the added sensor adds complexity, weight and volume. Ideally, the changing electrical properties of the SMA material itself can be used as a position sensor. But because the behavior of the SMA motion and electrical properties are nonlinear and exhibit hysteresis, it has proven difficult to create a robust self-sensing control scheme.
Several US patents disclose SMA actuators that use the electrical resistivity of the wire for sensing. The electrical resistance of an SMA wire or ribbon has been used as a position sensor in U.S. Pat. Nos. 7,886,535, 8,339,073. These methods measure the resistance of the entire length of SMA material, and anticipate the electrical resistivity of the entire material to change in proportion to position, stress or temperature. None of these methods can account for the fact that SMA electrical resistance is not a function of strain alone, but is also affected greatly by stress, fatigue, and ambient conditions.
Accordingly, there is a need in the art to provide an improved apparatus and method for determining a reliable and robust estimate of material length. It is to such that the present invention is directed.